1. Field of the Invention
The present invention relates to spindle motors mounted in such as information devices or acoustic devices which require precise rotation, and in particular to spindle motors to be used in HDDs (hard disk drives) in magnetic disk units.
2. Related Prior Art
Spindle motors for HDDs are ordinarily constructed with a hub on a rotating shaft rotated by an electromagnetic motor and supported on bearings so as to rotate freely. Known examples are disclosed in (1) Japanese Patent Application (Kokai) Hei 11-262214 and (2) Japanese Patent Application (Kokai) 2001-289242. Bearings suitable for these spindle motors comprise combinations of radial bearings and thrust bearings respectively supporting the radial loads and thrust loads of the rotating shafts. The radial bearings and the thrust bearings are fluid dynamic bearings supporting the rotating shafts with high precision because of the dynamic pressure action of lubricants.
In addition to high rotation precision, HDD motors mounted in personal computers and notebook personal computers require high quietness and shock resistance. It is also desirable to reducing thickness of the end-product. Fluid dynamic bearings are effective from the standpoint of ensuring quietness and rotation precision. The shock resistance is ensured by providing a fluid dynamic thrust bearing for positioning and restraining removal of the rotating shaft in the axial direction. The positioning in the axial direction is particularly important in HDD motors where magnetic disks have to have their positions set in the axial direction. However, in spindle motors furnished with thrust bearings, the rotating shafts sometimes tend to shift easily in the axial direction and separate from the thrust bearings because of the dynamic pressure force generated by the thrust bearing. In order to suppress this phenomenon to ensure proper positioning and to prevent the rotating shaft from moving in the axial direction, the following various means have been previously adopted.
(1) Staggering the magnetic centers of the motor stator and the motor magnet so that the shaft-carrying hub to which the motor magnet is attached is pulled downwardly in the direction of the thrust bearing.
(2) Positioning a magnetic plate opposite the motor magnet attached to the hub, to create a magnetic force by which the hub is attracted in the thrust bearing direction.
(3) Providing a permanent magnet, as described in prior patent cite, to attract the hub in the thrust bearing direction with the permanent magnet.
(4) Providing a flange at the end of the thrust bearing side in the rotating shaft, as described in prior patent (2), to prevent disengagement of the rotating shaft from the radial bearing. Also, in this construction, dynamic pressure force is generated between the flange and the radial bearing, so that the rotating shaft is pulled in the direction of the thrust bearing by this dynamic pressure force.
However, in the motor having staggered magnetic centers, an electromagnetic noise is generated resulting in a loss of quietness. The motor construction having a magnetic plate results in a loss of drive force in the motor, and this is particularly undesirable because it puts a limit on the value of current in devices where power is low as in notebook computers. With respect to motor constructions disclosed in patents (1) and (2), along with increase of the number of parts and complications in manufacturing, it is inconvenient to reduce thickness of the product utilizing these motors.
Also with respect to a rotation precision, when both the radial bearing and the thrust bearing are fluid dynamic bearings, the rotation precision is sufficiently ensured, but it has been difficult to construct HDD motors in which both the bearing stiffness and bearing loss were optimized, that is with both a high bearing stiffness and a small bearing loss.